Problem: What is the greatest common factor of $40$ and $20$ ? Another way to say this is: $\operatorname{gcf}(40, 20) = {?}$
Explanation: The greatest common factor is the largest number that is a factor of both $40$ and $20$ The factors of $40$ are $1$ $2$ $4$ $5$ $8$ $10$ $20$ , and $40$ The factors of $20$ are $1$ $2$ $4$ $5$ $10$ , and $20$ Thus, the greatest common factor of $40$ and $20$ is $20$. $\operatorname{gcf}(40, 20) = 20$